Combined electromagnetic and thermoresponsive protective device



- April 28,1931.

G. A. BURNHAM 1,802,804

COMBINED ELECTRO MAGNETIC AND THERMO RESPONSIVE PROTECTIVE DEVICE Filed May 11. 192.8

'Patented Apr. 28, 1931 UNITED STATES PATENT OFFICE GEORGE A. BURNHAM, OF SAUGUS, MASSACHUSETTS, ASSIGNOZR. T CONDIT ELECTRICAL IANUFAGI'UBJITG CORPORATION, OI SOUTH BOSTON, MASSACHUSETTS, A CORPORA- TION' OF MASSACHUSETTS COMBINED ELECTROMAGNETIC AND THERMORESPONSIVE PROTECTIVE DEVICES Application filed May 11, 1928. Serial No. 277,072.

'This invention relates to protective devices for electric circuits and particularly to electro-magnetic relays and similar devices associated with a circuit interrupter for the 6 purpose of opening the circuit interrupter upon overload.

Some electric circuits and particularly those feeding electric motors are subject to momentary overloads, as for instance, when 10 the interrupter is closed for the purpose of starting the motor. After the motor has come up to speed the current drops to some normal value. It is obviously undesirable to have the interrupter open under the momentary starting current. Heretofore overload protective devices have been provided with dash pots to hold the device inoperative for the time that the starting current flows in the current whereby to revent the opening of the interrupter. uch devices will not ordinarily give instantaneous opening of the interrupter if an abnormal overload, not occasioned by the starting of the motor, appears on the circuit of the interrupter.

It is an object of the present invention to provide an overload responsive device that will operate instantly on high overloads and will have a time delayed operation for less intense overloads.

A further object of the invention is the .provision of an overload responsive device of the electro-magnetic type wherein the setting of the movable member is automatically varied by the overload current and the time that the overload current persists on the circuit.

Another object of the invention is theprovision of an overload responsive device of the electro-magnetic type wherein the length 40 of the air gap in the magnetic circuit is controlled by the strength of current in the circuit to be protected and the length of time that an overload exists on the circuit.

Another object of the invention is the provision of a device of the class described wherein the armature of the electro-magnetic device is moved toward the electro-magnet by a thermally expansible member which is heated by current derived from the circuit to be protected, whereby to reduce the current responsive setting of the armature and after a time delay to cause it to operate at a reduced current value.

A further object of the invention is generally to improve the construction of overload responsive electro-magnetic devices.

Fig. 1 is an elevation of a device embodying the invention adapted for use as a relay.

Fig. 2 is a diagram of an electric circuit to be protected withthe device of Fig. 1 associated therewith.

Fig. 3 is a circuit diagram illustrating a modified form of the device of Fig. 1 wherein the device is adapted directly to open the circuit interrupter upon overload in the circuit thereof.

The device embodying the invention comprises a U-shaped electro-magnet 10 having an armature 12 pivoted thereto by the pin 14. An energizing coil or winding 16 is arranged on one of the legs of the electromagnet. The armature is adapted for vertical movement toward the electro-magnet when the coil 16 is energized and when the coil is de-energ ized to fall by gravity to an unoperated position against the stop member 18. Said stop member is adjustable toward and away from the armature whereby to vary the positionof the armature to be attracted by the electro-magnet for various values of current traversing the energizing winding 16. Thearmature carries a plate 20 provided with an insulating post 22 which carries a bridging member 24. When the armature is in an elevated or attracted position, said bridging member is adapted.- to engage and electrically connect a pair of contact members comprising posts 26, thus to complete a relay circuit including said posts.

In accordance with this invention, means are provided, influenced. by the current in the circuit to be protected, automatically to vary the setting of the armature 12. Said means include a thermo-responsive member comprising the bi-metallic strip 28 which is composed of two metals of dissimiliar coeflicients of expansion secured together back to back. One end of said strip is fixed to a bracket 30 and the other end 32 is free to move. A heating coil 34: is located in heat transferring relation with said thermostatic strip.

As here shown, the coil is wound upon said strip and is connected in series with the energizing coil 16 of the electro-magnet. A screw 36 is adjustably screw threaded in the free end 32 of said bi-metallic strip and is swivelled to the upper end of a lever 38 pivoted at 40 to the bracket 30. The lower end of said lever 38 is pivoted at 42 to the upper end of a link 44;. The lower end of said link overlies the end of the bar 20 carried by the armature 12 and has a longitudinal slot 46 therein in which a pin 48 carried by said bar 20 is loosely received. The arrangement is such that when said bimetallic strip 28 becomes heated due to the current carried by the heating coil 34 the free end 32 of the strip moves outwardly or toward the right and thus raises the armature 12 off the stop member 18 and into an elevated position nearer the electromagnet 10 than the normal position of said armature against the stop member 18. The armature will thus be attractedby the electro-magnet to bridge the contact members 26 at a lower current value than that provided by the setting of the stop member 18. The armature is free to move upwardly independently of the thermostatic strip by the lost motion connection provided by the slot 46.

As thus arranged, when a moderate overload traverses the circuit from which the energizing winding 16 and heating coil 34 derive their energy, the armature 12 will not be effectively influenced by the electromagnet. The thermostatic strip 28 will become heated however and expand and after a time delay determined by the characteristics of the device, the armature 12 will be elevated off the stop member 18, thus shortening the air gap in the magnetic circuit and bringing the armature sufliciently into the field of the electro-magnet 10 to cause the armature to be operated thereby; If the overload is very great, the armature 12 will be attracted by the electro-magnet independently of the action of the thermo-responsive device.

In Fig. 2, the electro-magnetic device is associated with the motor circuit 50 controlled by the circuit interrupter 52. The device is energized from said circuit in any suitable manner as, for instance, by the cur rent transformer 54. The contact members 26 of the device control the tripping relay 56 which operates when energized to trip the latch 58 of the interrupter and thus open the circuit.

In Fig. 3, the plate 20 of the device is arranged directly to engage and trip the latch 58a of the interrupter 52a.

I claim:

1. An electro-magnetic overload protective device for an electric circuit including an inverted U-shaped electro-magnet, an armature therefor having a pivotal support at one end and normally positioned out of the field of said electro-magnet, a contact bridging member carried by the free end of said armature, cooperating spaced stationary contacts arranged to be bridged by the bridging member of said armature 1n the attracted position thereof, means to move said armature into the field of said electromagnet including a toggle, and operating means for said toggle including a thermallyexpansible element responsive to overload current in the electric circuit to be protected.

2. An electro-magnetic overload protective device for an electric circuit including an inverted U-shaped electro-magnet having an energizing coil on a leg thereof, an armature pivoted to the other leg thereof and biased by gravity into a position below and out of the normal field of said electro-magnet, a pair of spaced stationary contact members, a cooperating bridging contact member carried by the free end of said armature and arranged in the attracted position of said armature to engage and bridge said stationary contacts, means including an overload-responsive thermally-expansible member to move said armature toward and into the field of said electro-magnet, and force multiplying means connecting said thermally-expansible member and armature.

3. An electro-magnetic overload protective device for an electric circuit including an inverted U-shaped electro-magnet having an energizing coil on a leg thereof, an armature pivoted to the other leg thereof and biased by gravity into a position below and out of the normal field of said electro-magnet, a pair of spaced stationary contact members, a cooperating bridging contact member' carried by'the free end of said armature and arranged in the attracted position of said armature to engage and bridge said stationary contacts, means including an overloadresponsive thermally-expansible member to move said armature toward and into the field of said electro-magnet, and force-multiplying means connecting said thermallyexpansible member and armature, the connection between said thermally-expansible member and armature also having provision for movement of said armature into attracted position free of said thermo-responsive member.

4. An electro-magnetic. overload protective device for an electric circuit including an inverted U-shaped electro-magnet having an energizing coil on a leg thereof, an armature pivoted to the other'leg thereof and biased by gravity into a position below and out of the normal field of said electro-magnet, a pair of spaced stationary contact members, a cooperating bridging contact member carried by the ree end of said armatureand arranged in the attracted position of said armatureto engage and bridge said stationary contacts, means including an overload-responsive thermally-expansible member to move said armature toward and into the field of said electro-magnet, and force multiplying means connecting said thermally-expansible member and armature, said armature having a lost-motion connection with said force multiplying means whereby to permit movement of said armature in the direction of said electro-magnet free 2 from said thermally-expansible member and also free from said force-multiplying means.

5. An electro-magnetic overload responsive device for an electric circuit including an electro-magnet having a depending core I member, an armature having a pivotal support at one end positioned below said core member and disposed in a remote region of the field of said electro-magnet whereby to be normally unattracted thereby, circuitcontrolling means carried by the free end of said armature, a projection at the other and fixed end of said armature extended beyond its pivotal support, a force multiplying device comprising a toggle having a lost motion pivotal connection at one end with the extended end of the said projection on said armature, a thermally-expansible element fixed at one end and having its free end pivotally connectedwith the other end of said toggle. and arranged when heated to expand in a direction to straighten said tog 1e and movesaid armature into the field of said electro-magn'et, and a heating element adapted to heat said thermally-expanslble element.

6. An electro-magnetic overload responslve device for'jthe' protection of an electric circuit including an electro-magnet having a depending magnetic core, an energizing wlnding therefor energized from the cir cuit to be protected, an armature for said electro-magnet positioned below and at a distance from said core whereby to beat tracted only upon a predetermined overload o energization of said winding, time consuming overload responsive means influencedby aoverlo'ads less than said predetermined overload value, and force multiplying means including a thrust toggle having a connection at its free end with said armature and having an operative connection with said overload responsive means whereby to straighten 

